1. Field of the Invention
This invention provides an attitude sensing electrical switch that uses environmentally safe powders as a switching medium.
2. Brief Description of the Prior Art
Attitude sensing or tilt switches are used widely in automotive applications and home equipment to complete an electrical circuit when the switch is moved into a predefined attitude. The switches are used extensively in automobiles to turn on trunk lights and underhood lights when access doors to those spaces are opened. An attitude sensing switch also is used in many home thermostats to complete or interrupt electrical circuits that control heating or cooling equipment when a bimetal device moves the switch in response to temperature changes.
Most attitude sensing switches use mercury as the switch medium. Mercury is a liquid at all temperatures reasonably encountered by automobiles and trucks and within homes, and it is a ready conductor of electricity. The auto industry uses nearly ten tons of mercury each year in attitude sensing switches.
Mercury is a naturally occurring mineral that does not degrade and is not destroyed by combustion. When released in vapor form to the atmosphere, mercury is redeposited on land and water surfaces where a portion is converted into methylmercury. This compound accumulates in aquatic organisms, enters the food chain, and eventually is ingested by humans. It has toxic effects on living systems and has been found to be a neurotoxin that can damage the central nervous system of humans.
Efforts to reduce the emissions of mercury are underway on a number of fronts. Waste containing mercury is classified as hazardous under the Resource Conservation and Recovery Act and is subject to careful disposal controls. Whether these controls will be adequate to protect human health remains to be seen, and efforts to reduce the uses of mercury are underway on several fronts.
This invention provides an attitude sensing electrical switch that uses electrically conducting powder as the switching medium in place of mercury. The switch comprises a housing made of an electrically conductive or an electrically insulating housing material. An electrically conducting powder is movably located inside said housing. With a housing made of an electrically conductive material, an electrical terminal extends into the housing and is electrically insulated from the housing. When the housing is tilted into a first attitude, the conductive powder moves into a position where it provides an electrical connection between the housing and the electrical terminal. The powder moves into another position where it interrupts the electrical connection between the housing and the terminal when the housing is tilted into a second attitude.
The electrically conductive housing can be made of brass, aluminum, copper, or other materials. The housing preferably is cylindrical and the electrical terminal is located in a plug made of an electrically insulating material that closes one end of the housing. The housing can be made by a stamping process so that the other end is closed and smoothly rounded.
Conductive powders with low electrical resistance and good flowing characteristics are preferred. Silver, gold, copper, beryllium, rhodium, iridium, and tungsten powders with particle sizes of seventy-five to three hundred microns perform effectively. Powders with larger particle sizes of one-hundred-forty to three hundred microns generally have better flowing characteristics and are preferred. Filling the empty space of the housing with an inert gas such as argon or nitrogen helps extend the useful life of the switch.
In the alternative structure in which the housing is nonconductive material, two electrical terminals extend into the interior of the housing and a quantity of an electrically conducting powder is movably located inside said housing. In the manner described above, tilting the housing into a first attitude enables the conductive powder to flow into a position where it provides an electrical connection between the two electrical terminals. Tilting the housing into another position enables the powder to flow into a second attitude where it interrupts the electrical connection between the terminals.
The nonconductive material for the housing can be tubing made of glass or a non-sticking polymeric material such as polyamide or polyfluorcarbon. Several different structures can be designed to utilize the invention. Both terminals can be embedded in the cylindrical wall of the housing, both terminals can be included in plugs that close the ends of the housing, or a combination of these can be used.